Estradiol (E2) has been repeatedly shown to be positively associated with breast cancer risk. Most E2 is converted into relatively inert metabolites. It is conceivable that the risk of breast cancer may differ among women with various patterns of estrogen metabolism. The investigator proposes a study to test this hypothesis in a case-control design of 40 newly-diagnosed early-stage breast cancer patients and an equal number of controls. All cases and controls will be identified among women who undergo a routine breast cancer mammographic screening in selected hospitals in the Minneapolis, St. Paul twin cities areas. Overnight urine samples will be collected to measure selected hormone metabolites. An in vivo urine caffeine test will be performed to determine the activity of estrogen-metabolizing enzymes. The above data collection will be repeated for all cases and controls 3 months after the breast cancer surgery of the index case to assess possible influence of breast cancer surgery and disease sequelae on the levels of these biomarkers. In addition, intra-variability of estrogen metabolizing pattern will also be assessed in this study. The information obtained from this study will be used to assess the association between estrogen metabolizing pattern and breast cancer risk.